Yes
Officially only on fire roads. Start at Alhambra entry, Briones Rd, or Bear Creek. The "good stuff" is all single track that's not marked. Look for the single track that suddenly appears at the top of the ridge line. It's not for beginners.
Yes. Many trails will accept bikes
Yes on biking trails. I usually look up all trails app to see which route is suitable
With courtesy to the walkers?
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